Bomb-sniffing technique creates a laser beam out of thin air

Engineers at Princeton University have developed a technique to create highly sensitive lasers from
thin air, which would increase the reliability and accuracy of
pollutant-detecting beams.

Being able to determine if there are gases in the air, from a
distance, can be extremely important if those chemicals are
dangerous or even deadly. Scientists need a way to sample
greenhouse gases and pollutants in the atmosphere without being
subjected to those harmful chemicals, and soldiers need to sniff
out an explosive chemical without putting themselves in the
blast radius.

Traditional laser-sensing methods, like LIDAR
(light detection and ranging), measure the scattering of a beam as
it bounces back from a distant object and hits the sensor on its
return journey.

LIDAR is used to measure cloud density or air pollution, but
it can't determine the actual identity or location of particles in
the air. Meanwhile, specially-crafted lasers that can identify
those contaminants can be insensitive and inaccurate, and are
unable to detect trace amounts or determine the location of those
gases with much precision.

So instead of relying on that same, unreliable beam of light
being bounced back, this Princeton-developed technique sends out
one laser, which then forms an entirely new beam from the oxygen
atoms in the air on the way back.

An ultraviolet laser pulse is focused on a tiny patch of
air, much like how a magnifying glass can focus sunlight with such intensity that it can start a flame. Oxygen
atoms in this one millimetre wide region get "excited" as their
electrons see rapid spikes in energy levels. When the laser pulse ends the electrons fall back down and emit infrared
light, which transfers to the next atom. Soon, a domino effect is
created, forming a beam that goes back to the sensor.

"The returning beam interacts with the molecules in the air
and carries their finger prints," says Richard Miles, a professor
of mechanical and aerospace engineering at Princeton. This can tell
the laser operator exactly what chemicals are floating in the air,
and their precise location.

Right now, the process can be replicated in the laboratory
over a distance of about a foot and a half. However the engineers
plan to increase the distance and fine-tune the sensitivity to pick
up very small amounts of airborne particles. "We'd like to be able to detect
contaminants that are below a few parts per billion of the air
molecules," Miles said. "That's an incredibly small number of
molecules to find among the huge number of benign air
molecules."

The research, which was funded by the Office of Naval
Research's basic research programme on Sciences Addressing
Asymmetric Explosive Threats, was published in the journal Science.